Brakes, such as those used on aircraft wheels, often include a brake disk stack comprising a plurality of rotors connected to a wheel which rotors rotate between a plurality of stators fixed to a torque tube on a wheel support. An actuator is used to compress the brake disk stack and bring the rotors and stators into contact in order to slow the wheel.
In electric brakes, the actuator is an electromechanical actuator having a piston that is driven toward and away from the brake disk stack by an electric motor. It is generally necessary to maintain power to the electric motor to hold the actuator against the brake disk stack with a given braking force. However, if a braking force needs to be held for a relatively long time, while an aircraft is parked, for example, a continued application of pressure by the piston wastes energy, generates excess heat, and may prematurely wear out the electric motor.
It is known that a parking (or long term) brake function can be achieved with electric brakes by applying pressure against the brake disk stack and then mechanically locking the shaft of the motor that drives the actuator. Power can then be removed from actuator, and the locked motor shaft will prevent the actuator from moving. In this manner, the wheels of an aircraft can be locked for relatively long periods of time, overnight, for example, using little or no power to maintain a required braking force.
The temperature of an aircraft brake system increases significantly after a landing, and this temperature increase causes the heated parts of the brake system to expand. As heat from the brake disks is transferred to the torque tube supporting the stators, the torque tube heats and expands, carrying the brake disk stack away from the piston. Therefore, if the piston is locked in place shortly after a landing, the brake force generated by the piston will decrease for several minutes thereafter as the torque tube heats and expands and moves away from the piston. Then, as the torque tube cools, it will contract and move the brake disk stack back toward the piston. Since the piston was locked in place when the torque tube was hot, when the torque tube cools it will press the brake disks against the piston with a great force and may damage or destroy the brake and actuator.
This problem can be avoided by maintaining power to the brake controller and continuously measuring the force exerted by the piston on the brake stack (or vice versa) and adjusting the position of the piston in order to maintain the brake force between a given minimum and maximum. Alternately, to conserve power, the control system may be shut down and then periodically powered up to determine whether a change in brake piston position is needed. However, it may take several hours for aircraft brakes to fully cool, and thus the control system must stay active, or be repeatedly reactivated, for a period of several hours. It would be desirable to provide a system for adjusting brake piston position in response to brake temperature changes that will accommodate brake heating and cooling after a landing while at the same time conserving power and minimizing involvement of a control system.